In recent years, high-performance solid-state imaging devices using semiconductors have been mounted in cellular phones, digital video cameras and the like, and the number of pixels and performance such as sensitivity has been dramatically improved. The solid-state imaging devices are manufactured by forming a circuit constituted by photodiodes and the like in an epitaxial layer, for example, using an epitaxial substrate in which the epitaxial layer is grown on one surface of the semiconductor substrate.
In recent years, as miniaturization and increasing resolution of the solid-state imaging device have progressed, the layout density of the photodiodes has considerably increased. For this reason, since the sizes of the individual photodiodes become extremely small, the amount of light capable of being incident to each of the photodiodes decreases. In order to avoid a decrease in the amount of incident light due to progression of miniaturization and increasing resolution of the solid-state imaging device, back illuminated solid-state imaging devices having a structure in which light is incident from the back side thereof with a small number of constituent parts such as a circuit layer are also generally known.
However, a dark leakage current of the photodiode is a problematic factor which causes deterioration in the imaging characteristics of the solid-state imaging device. It is considered that the dark leakage current is caused by heavy-metal contamination of a substrate (wafer) during the manufacturing process.
Hitherto, in order to suppress heavy-metal contamination of the substrate, operations have been performed for reducing the concentration of heavy metals in the photodiode forming portion by forming a gettering sink for heavy metals in the inside or the back surface of the semiconductor wafer and collecting heavy metals in the gettering sink.
In addition, recently, as cellular phones, digital video cameras and the like have considerably thinned, the thinning of a semiconductor device, for example, a semiconductor memory built into these apparatuses has progressed. The semiconductor memory is manufactured by forming a device on one surface of a silicon substrate (silicon wafer) made of, for example, a silicon single crystal. In order to make the semiconductor memory thin, the device is formed on the surface side of the silicon substrate, and then the device is thinned to a thickness of, for example, 50 μm or so, by cutting the back side of the silicon substrate.
In such a step of thinning a semiconductor device, there is a growing concern about mixing of heavy metals into the silicon substrate. When impurities such as heavy metals are mixed into the silicon substrate, the device characteristics are remarkably deteriorated due to the leakage current and the like. For this reason, it is important to suppress dispersion of heavy metals in a device forming region after the step of thinning the silicon substrate.
Hitherto, as a method of removing heavy metals from the silicon substrate, a gettering method is generally known. This is to reduce heavy metals in the element forming region, by forming a region for capturing heavy metals, referred to as a gettering site, in the silicon substrate and collecting heavy metals in the gettering site using an annealing treatment and the like. As a method of forming the gettering site in the silicon substrate, for example, an IG (intrinsic gettering) method of forming oxygen precipitates in the silicon substrate (for example, PTL 1), and an EG (extrinsic gettering) method of forming the gettering site such as back side damage in the back side of the silicon substrate (for example, PTL 2) are known.